Enhanced valley polarization in MoS ₂ via substrate-induced strain and transition dipole moment modulation

Monolayer transition metal dichalcogenides (TMDs) are promising materials for valleytronic and quantum photonic applications due to their valley-selective optical transitions. However, tailoring and enhancing valley polarization remains a challenge, particularly under ambient conditions. Here, we present a comprehensive helicity-resolved femtosecond transient absorption spectroscopy (HR-TAS) study of large-area monolayer MoS₂, comparing the as-grown and substrate-transferred samples to uncover the impact of strain and defect engineering on valley depolarization dynamics. Our measurements reveal that the degree of polarization (DOP) and the valley lifetime are significantly enhanced in transferred MoS₂, with a pronounced dependence on excitation fluence and temperature. We attribute this enhancement to a combination of strain-induced symmetry breaking, defect-induced carrier localization, and reduced intervalley scattering via the Maialle-Silva-Sham mechanism. Supported by first-principles calculations, we identify a novel contribution from spin-dependent transition dipole moments (TDMs), which further boosts valley selectivity in strained MoS₂ on quartz substrates. This study provides an experimental and theoretical correlation of TDM modulation with enhanced valley polarization in transferred MoS₂. Our findings offer a pathway to enable new design principles for spin-valley-coupled optoelectronics applications through substrate and strain engineering.